Winches, such as are used on material handling carriers, hoisting equipment, overhead cranes and the like, have been susceptible to undesirable cable unraveling from the winch drum after the load has come to rest if the winch drive motor is not promptly turned off by the machine operator. Normally the cable is wrapped around the winch drum in spiral grooves. If tension on the cable is relaxed and the drum motor continues to rotate in the unwinding direction, the wraps of cable can separate from their grooves and overlay upon themselves, resulting in cable wear and, often, improper function of the winch when the next load needs to be lifted. Further, such unwinding often results in the cable becoming entangled with other portions of the machine structure which can lead to cable breakage or injury to the operator. These problems are particularly serious when the carrier is equipped with multiple winches in that malfunction of one or more winches may result in the load being tipped or spilled.
Certain types of apparatus are known to the art for disabling a winch if a slack cable condition exists. One type employs a traveling-nut limit switch, which has a threaded shaft mechanically coupled to the rotating shaft of a winch drum so that the two shafts are colinear and rotate coincidently. A threaded nut is mounted on the limit switch and moves back and forth along the length of the shaft as the shaft rotates. Near one end of its path, the nut engages an interface device, usually an electrical contact set, which is thereby repositioned to de-activate the winch motor and preclude further unwinding of the cable. The position of the electrical contact set is adjustable with respect to the traveling nut to effect repositioning of the contacts by the traveling nut at the moment the load comes to rest and the cable starts to become slack.
There are several disadvantages to apparatus of this type, one being that it can not compensate for elongation or stretching of the cable which commonly occurs after extended use. Once the cable has stretched, a slack condition and cable unraveling can occur before the traveling nut activates the electrical contact set. Another disadvantage of such an apparatus is that, of necessity, it must be preset for a particular assumed load platform height. If the machine operator tries to deposit a load into, say, a pit below this assumed height, the traveling nut device causes premature shutoff of the winch. On the other hand, if the operator attempts to deposit the load above the assumed height, a slack cable condition and cable unraveling can occur well in advance of the shut-off point preset within the traveling nut apparatus.
Another sensing apparatus known to the art includes a loop or bale formed of rigid rod or wire surrounding the cable at a point above the hook block and just below the lifting carriage. This bale is attached to an interface device, usually an electrical contact set, by means of a rigid lever support member. Slackening of the cable causes a lateral displacement of the bale and movement of the lever support in a manner to reposition the interface device, precluding further operation of the winch in the unwinding direction. One disadvantage of this type of apparatus is that a slack cable condition adjacent the hook block may cause the winch to stop even though the cable at the winch drum itself has not become slack, e.g. as may be caused by friction in the reeving pulleys or for other reasons. The result, of course, is a false signal. Another disadvantage of an apparatus of this type is that, of necessity, it must be mounted in close relation to the vertical portion of cable between the hook block and the lifting carriage. If the lifting carriage is of the traversing type and if the interface device is of the electrical type, an electrical collector system must be employed to transmit the signal from the lifting carriage to the winch control system, the former being movable with respect to the latter. Electrical collector systems increase expense of fabrication and typically require significant maintenance to assure system integrity. Yet another disadvantage of an apparatus of this type is that an attempt to lift a load which is off-center with respect to the lifting carriage vertical centerline will often result in false signaling caused by the angle at which the cable is caused to pass through the bale.